The present invention generally relates to the magnetic particle testing area of non-destructive testing and, more particularly, to an apparatus which assures proper mixing of the magnetic particle bath suspension at all times.
Magnetic particle testing is a well known method of nondestructive testing which is used to indicate defects in ferromagnetic materials. These defects may be detected by magnetizing a part to cause localized magnetic leakage fields to occur at points on the surface of the part where there are cracks or the like forming magnetic discontinuities. Such leakage fields, and therefore cracks, can then be detected by dispersing finally divided composite magnetic particles over the surface of the part which will concentrate at regions where the leakage fields are produced. Normally, dispersement of these fine particles is accomplished in what is known as a "wet" process by dipping the entire part into a suspension, or spraying the suspension into the parts, which is composed of either an oil or water carrier and these fine composite magnetic particles.
Such testing is very successful but relies on the proper concentration of fine composite magnetic particles and carrier within the suspension to insure accurate results. If the proper concentration is not maintained, then the sensitivity of the test is decreased when the suspension has a low magnetic particle concentration and too much background is present when it has a high concentration. Moreover, since the composite magnetic particles are more dense than the carrier, they readily settle to the bottom of the suspension. Therefore, it is also imperative that the magnetic particle suspension be properly mixed on a continuing basis to insure proper results.
Existing magnetic particle testing systems rely on a pump to maintain proper mixing of the suspension as it circulates through the system. This system is undesirable not only because proper mixing is not maintained but, more importantly, the force of the pumps impeller produces too much shearing action for the magnetic particles. This is critical because the composite magnetic particles are actually composed of a fine magnetic particle which is attached to a pigment in a complex process. These pigments provide a visual indication of a defect in an object after the magnetic leakage fields attract the magnetic particle portion of the composite magnetic particle. Therefore, the presence of excessive shear forces in the mixing system causes the pigment to separate from the magnetic particle thereby reducing the effectiveness of the test.
The apparatus of the present invention overcomes these problems by providing a unique suspension mixing system which relies on gravity and air, injected within the suspension, to provide proper suspension mixing without any shear forces to separate the magnetic particles. In this system, air is injected through an inlet valve into the bottom of a mixing tank containing a desired concentration of suspension. Dispersed within the center of the tank is an elongated hollow tube which is positioned slightly above the inlet valve to channel the air, now in a bubbling form, to the top of the suspension. As the air enters the bottom of the tank, it contacts the suspension before it enters the tube and pushes the suspension up the tube. After reaching the top of the tube, the air escapes to the atmosphere and the suspension from the bottom now exits into the top of the suspension where the composite magnetic particles begin to settle to the bottom again, thereby providing a constant flow of suspension within the tank.
To prevent settling and sticking of the magnetic particles to the bottom of the tank, the tank is first provided with a conical bottom. Secondly, a conical or umbrella shaped member is affixed along the length of the hollow tube with its large open end facing the bottom of the tank at a position slightly above the conical shaped tank bottom. Finally, small "scrubber" beads, having a density greater than the composite magnetic particles, are introduced into the suspension. These beads are circulated in the same way the magnetic particle suspension is. However, after falling back through the suspension, they roll down the outer side of the umbrella shaped member as well as the sides of the conical bottom to further mix the magnetic particles and prevent their settling and sticking to the bottom of the tank.
It is therefore an object of the present invention to provide a proper system and apparatus for mixing magnetic particle suspension.
It is a feature of this invention to have scrubber beads within a suspension which are circulated along with the suspension by rising air injected into the bottom of a mixing tank.
It is an advantage of the present invention that the system does not impart shearing forces within the suspension while properly mixing the magnetic particles of the suspension and preventing particles from settling on the walls of the mixing tank.